1. Field of the Invention
The present invention relates to a method for recycling rubber scrap, primarily tire tread buffings, to obtain a final recycled rubber product of at least about one inch in thickness. The method is characterized by its use of only dry ingredients which are intimately mixed, formed in a mold and cured into a steam autoclave. The final recycled rubber product actually wears and weathers better than new rubber and may be formulated in various hardnesses from about 55 to about 90 as measured on the Shore A scale. Products formulated according to the method of this invention are suitable for a variety of uses such as, for example, paving or stepping stones, floor coverings, resilient bumpers for highway guard rails, railroad tie pads, etc.
2. Background of the Invention
Numerous methods and processes for reclaiming vulcanized rubber scraps are known in the prior art. Perhaps the earliest patent for a reclaiming process was British Pat. No. 11,147 to Parkes, issued in 1846. That patent teaches that scrap rubber may be heated under pressure with calcium hypochlorite until the mass could be kneaded. This mass was then washed with alkali and hot water. U.S. Pat. No. 22,217 to Hall discloses a heater process for reclaiming rubber wherein finely ground rubber is mixed with a small quantity of softener and with caustic soda followed by heating in horizontal autoclaves for several hours in steam at temperatures from 347.degree. to 410.degree. F. An acid reclamation process is disclosed in U.S. Pat. No. 249,970 to Mitchell wherein the rubber scrap is first treated with a 10-25% sulphuric acid. An alkali process is taught by U.S. Pat. No. 635,141 to Marks, et al., and for a long time this was the most important commercial process for reclaiming rubber. A similar method for reclaiming rubber is taught by U.S. Pat. No. 673,057 to Cutler. Both the Cutler process and the Marks alkali process require the use of reclaiming oils. Other methods utilized for reclaiming rubber have been described in the literature as the acid process, the superheated steam process, the high pressure steam process, and the catalytic processes, in addition to mechanical processes using internal mixers and special extruders.
However, the literature observes that the physical properties of reclaimed rubbers are distinctly inferior to those of ordinary rubber and, in practice, such reclaimed rubbers are only used as fillers or to facilitate certain manufacturing operations such as extrusion. J. LeBras, INTRODUCTION TO RUBBER at 88 (1965). D. Huke states that most reclaimed rubber is obtained from tires, and that this rubber is treated so that it can be re-milled and incorporated into a rubber mix. D. Huke, INTRODUCTION TO NATURAL AND SYNTHETIC RUBBERS at 76 (1961). In fact, virtually all the prior art literature concerning reclaimed rubber and methods for reclaiming concern themselves with obtaining a "de-vulcanized" product for use as a filler/additive/extender in subsequent manufacturing operations. Also, as shown above and as is evident from a review of the pertinent prior art literature, today's processes for reclaiming rubber involve treating the rubber scrap with alkali, acid, solvents, superheated steam, mechanical agitation and shearing, or combinations of the above. For example, one of the more modern processes for reclaiming cured rubber scraps is disclosed in U.S. Pat. No. 4,051,080 to Boccoli. It is therefore apparent that there is a great need in the art for a method of not merely "reclaiming" rubber as taught in the prior art but of "recycling" rubber wherein operating conditions of the method are simplified and do not require treatment with alkalis, acids or oils. It would furthermore be desirable if the method could be accomplished so as to derive a final recycled rubber product suitable for commercial use, per se, without need of any further manufacturing process. It is therefore desirable that any such method would be sufficiently variable so as to provide final products of varying sizes, shapes, and degrees of hardness, such as highway guard rail extension blocks which hold the guard rails about eight inches from their vertical support posts. It was primarily as a result of efforts to produce such a product that the subject matter of this patent was developed.